1. Field of the Invention
The present invention relates to mobile, offshore self-elevating "jack-up units" or rigs for offshore oil work and more particularly to a system for making such a unit with its support legs rigid and fixed, when the legs are either up or down in a desired position, counteracting the major loads which these units must accomodate, namely fixed weights, variable weights, wind, currents and waves.
A "jack-up unit" as used herein means any working platform used for drilling, work over, production, crane work, compressor stations, diving support or other offshore purpose in an elevated position above the water, and being supported on jackable legs to the ocean floor or other water bottom, with the inherent capability of relocating from one site to another by lowering to a floating position, and, after being moved to a new, established location, raising again to an elevated position.
The present invention is intended to apply to any jack-up rig unit which is raised or lowered with a jacking apparatus, a typical example of which is disclosed in U.S. Pat. No. 3,606,251, or other pinion driven systems, that engage rack teeth on the legs.
2. Prior Art
Jack-up units equipped with rack and pinion type jacking systems have long been known as shown, for example, in U.S. Pat. Nos. 2,308,743 issued Jan. 19, 1943 to W. P. Bulkey et al; 3,183,676 issued May 18, 1965 to R. G. LeTourneau; and 3,606,251 originally issued Sep. 20, 1971 and reissued as U.S. Pat. No. 29,539 on Feb. 14, 1978 and owned by the Armco Steel Corporation. These units use the pinions to transfer the loads from the hull into the leg chords and vise versa, in conjunction with a guidance system required to take amounts due to wind, waves or other imposed loadings. The pinion supported units of U.S. Pat. No. 3,183,676, impose a horizontal component of the load transfer, due to the tooth pressure angle that directly imposes a moment in the leg chords. The units supported by the pinions in conjunction with a guidance system, have an inherent flexibility in the pinion gear train system that further introduces a moment in the leg chords through their guidance system.
To overcome this problem the present invention uses rack engaging members that engage, interdigitate and lock into preferably a number of the rack teeth of each leg. The "rack chock" horizontal contact with the leg chord rack bar is maintained by additional chocks, screws, wedges, etc., and the "rack chord" leg sections may be of numerous types.
Typical examples of some prior patents which show some form of other of leg teeth engaging devices in jack-up units are presented below:
______________________________________ Inventor(s) Patent No. Issue Date ______________________________________ S. Lewis 103,899 June 7, 1870 B. Laffaille 2,540,679 Feb. 6, 1951 C. A. D. Bayley 2,862,738 Dec. 2, 1958 A. L. Guy, et al 2,954,676 Oct. 4, 1960 G. E. Suderow 3,007,317 Nov. 7, 1961 L. J. Roussel 3,109,289 Nov. 5, 1963 J. L. Roussel 3,171,259 March 2, 1965 R. D. Yeilding 3,290,007 Dec. 6, 1966 Itoh, et al 3,722,863 March 27, 1973 Lucas 3,876,181 April 8, 1975 James Humby, et al Great Britain 934,369 August 21, 1963 ______________________________________
However, these patents do not fairly teach or suggest the present invention and are clearly distinguishable from the over-all rigidification system of the present invention.
3. Discussion of Forces Involved and Prior Art Systems
The present invention does not introduce any large secondary bending stresses than can limit the performance of the jack-up unit.
The graph, FIG. 8, "Operational Analysis--Variation of Stress Components in Critical Member", identifies the influence on leg stress when the "jack tower guides" of the prior art are used to take the leg moments. This method is used for most if not all existing designs, either entirely or in part to handle the leg moments. The dotted lines represent the leg axial stress that the system of the present invention absorbs directly; while the solid line represents the additional secondary bending stress due to the use of a "jack-tower" guide system.
Other prior designs use a rack and pinion system that has its line of support directed radially from the center of the leg. Due to the rack and pinion pressure angle, the leg receives a secondary bending load of approximately forty (40%) percent of the vertical loads.
These secondary bending stresses can be larger than the axial stress and have limited the potential for prior art jack-up units for going in to deeper waters and high wave sites.
To illustrate the basic development of forces and moments at the leg/hull interface, the simplified two-dimensional structural bent, as representing a typical leg/hull structure under environmental and weight loadings, illustrated in FIGS. 9A and 9B should be considered.
From the force pictures of FIGS. 9A and 9B the forces applied to that section of leg within or adjacent to the hull structure may be determined (taken above the wave zone, and for the more highly loaded leeward leg).
The forces in the leg just below the hull are seen with reference to FIG. 9B, to be directed almost entirely as axial loading in the chords, except for the nominal shear loading due to wind taken in the bracing. How these forces are taken in the hull depends primarily on the jack attachment as outlined below.
A. Resilient mounted jacks will deflect under load so that the leg will tend to rotate due to the overturning moment, and the guides will be required to resist some of this moment as a horizontal couple. In the extreme, with deep rubber pads that may deflect several inches or so, the jacks (pinions) will take only the vertical load imposed by the hull, 1/2(W/2+M/1), with the guides bearing all of the forces due to the moment, plus the wind shear. Thus, considerable stress must be borne by the bracings through this area, and in addition, the chords may be subject to large bending stresses (in additional to compression), particularly if the guides are at mid-bay.
As a result, not only is the leg extremely heavy, but the jack towers supporting the upper guides must also be substantial to carry the upper reaction load into the hull.
B. Jack fixed to the hull will tend to absorb directly almost all of the axial loading of the chords, including that due to the vertical couple of the overturning moment. Due to some torsional deflection of the pinion gear train (which is small) and the stiffness of the bracing (again which is small, relative to the chords), there will be some transfer of overturning moment as a horizontal couple at the guides. Generally, this will be small, and even with the addition of the horizontal wind shear, the bracing size will remain nominal (except where single pinion racks are used; see item 3 below).
However, since the jacks will take almost the entire loading of overturning in addition to the hull weight, it is probable that for severe environmental conditions, the number of pinions required will be greater than that needed for the sole purpose of jacking up or down. Pinion ratings for holding loads (with brakes set) are generally twice that permitted for normal jacking, and thus the load due to overturning would be limited to the same as that due to weight support, or additional pinions would be required.
C. Single Pinion Racks and Opposed Pinion Racks
Where rack/pinion arrangements provide only a one-sided rack with a sinle vertical line of pinions (note FIGS. 10A and 10B), such as for example is the case in the "Le Tourneau" type-rigs (a majority of all-present jack-up rigs), there is a large component of the pinion force directed horizontally that must be transmitted throuh the chord and bracing structure into the racking on the opposite chord. With the pressure angle of the rack teeth of 20 to 25 degrees being typical (note FIG. 10A), this horizontal force is of the order of 40% of the vertical force needed for rig support. This results in high bending stresses in the chords and high compressive stresses in the bracing, resulting again in an extremely heavy leg being required (whether the jacks are floating or fixed to the hull).
With opposed pinion racks (note FIGS. 11A and 11B), such as for example is the case in the "National" type rigs (approximately 10% of all present jack-up rigs), the horizontal forces are directly taken through the individual rack in compression (normal to the vertical compression and readily absorbed) and there is no input into the leg assembly.
The horizontal forces which increase the leg chord and bracing weight also require large size members for larger loads. These increases in turn incur larger waver loadings and then larger horizontal forces, etc. This "domino" effect has caused limitations on the capability of this prior art design unit.
The present invention outlined herein will eliminate the induced horizontal forces.
In the present invention, in for example the embodiment as developed for use with opposed racks (FIGS. 2-4), each of the "rack chock" elements of the system of the present invention is designed to absorb the maximum axial chord loading and transmit it directly into the hull. It is configured with preferrably a number of matching teeth for exact, in-line engagement with the legs' rack teeth, and is capable of being adjusted for vertical engagement to mate with the rack teeth position. By a series of screw jacks and/or secondary chocks, it will provide rigid contact with both the legs and the hull structure, and will eliminate the requirement for the jack pinions to take load, as is done in the prior art, in either jacked-up or ocean-tow dispositions.
Among the major advantages of the present invention are the following:
a. The legs will be of minimum scantling and weight, consistent with the design loads and environmental conditions, which, in addition to cost reduction, will provide greater capability under ocean tow conditions with legs raised and subject to roll dynamics.
b. There will be no need to provide additional pinions to take environmental loadings (for the case of jacks fixed to the hull). The jacks can be selected just for the service requirements of jacking up and down.
c. Pinions and their gear trains will not be subject to oscillating loads which cause wear and fatigue damage. This is of particular significance when under tow with high dynamic reversals of load.
d. With the rack chocks fully engaged in final position, complete jack assemblies may be removed for overhaul or replacement, or for use on another installation.
4. General Summary Discussion of Invention and Its Application
For purposes of this general discussion of the present invention, the legs of the rig are considered to be of the truss type, each leg having three or more chords and each chord incorporating, for example, a dual rack section having two opposed sets of rack teeth, each extending along one of the two edges of the rack bar. However, the present invention is applicable to legs of any structural form having any multiplicity of single or dual rack sections.
The present invention provides an improved method of rigidly supporting the "jack-up unit" in an elevated position on the legs of the unit, and/or of rigidly supporting the legs in a raised position when the unit is in an afloat disposition. In the invention the dual rack section is engaged with opposed, matching rack sections, which can be fixed to the unit. In the preferred embodiment, each matching rack section, called a "rack chock", can be adjusted vertically up and down along the legchord dual rack section and horizontally in and out to engage or disengage the leg chord dual rack section.
The "rack chock" of the rigidification system of the present invention transfers the loads from the hull into the leg chords or from the leg chords back into the hull. The "rack chock" elements accomplish at least in part this load transfer, and eliminate the introduction of moments in the leg chords which would otherwise occur due to the guidance system, or due to pinion reactions in the jacking system.
In the invention, the load transfer can be either through the "rack chocks" only or jointly with the pinions as desired.
The "rack chock" elements of the present invention utilize the necessary number of in-line tooth engagements to safely transfer the load and can have metalized tooth surfaces to distribute the load across the teeth evenly.
The "rack chock" elements can be engaged with the leg chord rack bar, pre-loaded to eliminate movement in the contacting tooth surfaces.
The "rack chock" elements of the ridification system can be moved vertically by mechanical or hydraulic means, such as for example, cylinders, screws, wedges, etc. The vertical positioning permits the indexing of the "rack chock" teeth with the leg chord rack bar teeth.
Each "rack chock" element can be fixed to the hull structure, after vertical positioning, by chocks, screws, wedges, etc. Fixing to the hull can be accomplished both above and below the "rack chock".
The horizontal movement to engage or disengage each of the "rack chock" elements with its respective leg chord rack can be by mechanical or hydraulic means, such as for example cylinders, screws, wedges, etc.
The "rack chock" horizontal contact with the leg chord rack bar is maintained by chock, screws, wedges, etc., and the "rack chord" leg sections may be of any numerous types.
With the use of the ridification system of the present invention the jacking systems are no longer needed to lock the legs in position and can be removed for use elsewhere, enhancing the economics of the invention. Additionally, with the availability of the present invention on a rig, it is estimated that perhaps as much as one thousand tons of steel can be saved in the fabrication of the rig. Also, with the present invention, it is believed that jack-up rigs will now have an extended range with respect to water and wave depths twice that it was before the present invention.